Tumor virus SV40 provides a unique opportunity for the study of the role of single genes in the initiation and maintenance of neoplastic cell transformation. It is an ideal virus for genetic manipulation and analysis since the virion contains only enough DNA to code for 4 to 8 proteins of average size. The virus may cause either productive infection of permissive monkey cells or transforming infection of restrictive mouse cells. The study of temperature-sensitive (ts) mutants of SV40 in monkey cells should better define the specific biochemical role of each viral gene in productive infection. The parallel study of the same mutants in mouse cells should indicate which of the viral genes are required to initiate and/or maintain transformation to the neoplastic state. An understanding of the specific genetic and molecular mechanism of virus-induced oncogenesis may produce insight into the mechanism of other forms of neoplasia as well. We have isolated more than 70 ts mutants of SV40 which are suitable for functional and genetic analysis. The mutants have been partially studied and found to represent 3 distinct functional groups. Two of these produce ts structural proteins in productive infection but transform restrictive cells normally. The third group is defective in the regulation of viral DNA replication, the control of late RNA transcription, the synthesis of structural proteins in productive infection and in initiating the transformation of restrictive cells. We propose to further define the specific function of each mutant group and to identify new mutant groups.